Efficient Plant October 2017 - 32

feature | reliability strategies

RESOURCES
Power Transformer Tank

Rupture: Risk Assessment
and Mitigation, EPRI TR104994, April 1995.

Proceedings: High Voltage
Current Transformers and
Bushings - Failure Prediction and Prevention Symposium, EPRI TR-113649,
Dec. 1999.

EPRI Substation Diagnos-

tics Conference VII - 1999,
A Condition Monitoring

System for Oil Filled Current Transformers.

Detection of Imminent

Failure of Oil Filled Cur-

rent Transformers - Final
Report, by Dan Bradley,
P.E., Bonneville Power

Administration, Proceed-

ings: High Voltage Current
Transformers and Bushings - Failure Prediction
and Prevention Sympo-

sium, EPRI TR-113649, December 1999, pages II-2-1
to II-2-16.

Electrical World T&D,

September/October 2000,
"Transformer Fires: Causes and Prevention," by
William Atkinson.

32

into Zone III, though, at some point the dielectric
eventually degrades to the point where it breaks
down electrically and suddenly allows arcing within
the HVCT. This arcing heats and vaporizes oil and
decomposes the oil vapor into gas. The resulting
gas bubble creates internal pressure and sometimes
causes the HVCT to rupture.
The exact point at which the dielectric fails while
in service and allows internal electrical arcing to
occur is difficult to predict. It depends upon various
factors affecting the oil, such as moisture content,
temperature, particle count, amount of metals in the
particles, gases in the oil, amount of added inhibitors, and oil acid number. Different combinations of
these factors change the point at which actual breakdown occurs. Prior to breakdown, changes in each of
these factors generally occur over time in a relatively
slow fashion. Once breakdown occurs, however, the
equipment will fail relatively quickly.
Using the condition-monitoring methods applicable to Zone II to predict when failure will occur in
Zone III is possible, of course, but involves more cost
and, frequently, more monitoring sensors and control equipment, and likely will require more people
resources. The question is, "How much is it worth to
your plant in risk and resources to operate right up
to the edge of failure?" Do you really want to replace
a major transformer, motor, or engine in the middle
of a production run or an operating cycle?
As a practical matter, replacement or refurbishment of significant production equipment should be
done while the equipment is still operating in Zone
II, with the time and place chosen by appropriate
plant personnel. Many operations essentially schedule end-of-life replacement of critical equipment to
coincide with an outage period just before the end of
Zone II, or perhaps even two outages before Zone II
ends. This type of scheduling provides flexibility for
dealing with unforeseen circumstance.
If that's how replacements or refurbishments are
scheduled at your plant, you need to know when
Zone II ends and Zone III begins. This can be
determined by consulting various databases with
statistical details on when equipment fails. EPRI
(Electric Power Research Institute, epri.com) offers a
fine database for most plant-equipment items, some

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of which are even subdivided by brand and model
number. (Many power plants base their preventive
and predictive maintenance programs on EPRI's
database.)
The current edition of the IEEE Standard 500
Reliability Manual is another good source, as are
publically available and tax-payer-funded military
and government publications such as MIL-HDBK338B, Electronic Reliability Design Handbook, ADA273 174, Naval Surface Warfare Center, Handbook
of Reliability Prediction. These and similar databases
are available on the Internet.
Finally, don't overlook accountants' amortization
tables for major equipment purchases. They also can
be helpful. One place to find them is in the U.S. Federal Government IRS Code, "Table of Class Lives and
Recovery Periods." (Banks don't like to lend money
on major pieces of equipment if the equipment is no
longer serviceable.)

KEEP IN MIND
When it comes to achieving reasonable reliability of
plant equipment:
 Don't try to predict failure in Zone III of the
bathtub curve by using condition-monitoring
methods more appropriate for Zone II.
 If at all possible, don't operate critical equipment in Zone III where predictability is dicey.
Avoidance of Zone III can be accomplished by
consulting various databases that provide a statistical basis for estimating useful service life. Choose a
confidence level appropriate to the level of risk your
organization is willing to accept with respect to
service life, and schedule replacement or refurbishment of the equipment accordingly. EP
Randall Noon is a registered professional engineer
and author of several books and articles about failure
analysis. He has conducted root-cause investigations
for four decades, for nuclear and non-nuclear power
facilities. Contact him at noon@carsoncomm.com.
Editor's Note: Along with the list of resources used
in the preparation of this article (left), additional
suggested reading can be found in the online version
at efficientplantmag.com.

OCTOBER 2017


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Table of Contents for the Digital Edition of Efficient Plant October 2017

Editorial
Implementations
Profitable Reliability Control On Your Horizon
Experts Provide IIoT Insights
Integrating Automation Into Shade Manufacturing
Fluid-Flow Challenges
Looking For Failure In All The Wrong Places
Closer Human/Robot Collaboration
Note These Causes Of Motor Failure
Reliable Pumping Supplement: Food & Beverage Processing
On The Floor
Uptime
IIoT
Pump 'Cavitation' Might Not Be
Never Too Late For Basic Navigation
Audit or Assess? Understand The Difference
A Perfect Pair: Barcodes and Your CMMS
Lubrication Strategies
Products
Ad Index
Showcase
Efficiency Insight
Efficient Plant October 2017 - 1
Efficient Plant October 2017 - Cover1
Efficient Plant October 2017 - Cover2
Efficient Plant October 2017 - 1
Efficient Plant October 2017 - 2
Efficient Plant October 2017 - 3
Efficient Plant October 2017 - 4
Efficient Plant October 2017 - 5
Efficient Plant October 2017 - 6
Efficient Plant October 2017 - 7
Efficient Plant October 2017 - Editorial
Efficient Plant October 2017 - 9
Efficient Plant October 2017 - Implementations
Efficient Plant October 2017 - 11
Efficient Plant October 2017 - Profitable Reliability Control On Your Horizon
Efficient Plant October 2017 - 13
Efficient Plant October 2017 - 14
Efficient Plant October 2017 - 15
Efficient Plant October 2017 - Experts Provide IIoT Insights
Efficient Plant October 2017 - Integrating Automation Into Shade Manufacturing
Efficient Plant October 2017 - 21
Efficient Plant October 2017 - 22
Efficient Plant October 2017 - 23
Efficient Plant October 2017 - 17
Efficient Plant October 2017 - 18
Efficient Plant October 2017 - 19
Efficient Plant October 2017 - Integrating Automation Into Shade Manufacturing
Efficient Plant October 2017 - 21
Efficient Plant October 2017 - 22
Efficient Plant October 2017 - 23
Efficient Plant October 2017 - 24
Efficient Plant October 2017 - Fluid-Flow Challenges
Efficient Plant October 2017 - 26
Efficient Plant October 2017 - 27
Efficient Plant October 2017 - 28
Efficient Plant October 2017 - Looking For Failure In All The Wrong Places
Efficient Plant October 2017 - 30
Efficient Plant October 2017 - 31
Efficient Plant October 2017 - 32
Efficient Plant October 2017 - 33
Efficient Plant October 2017 - Closer Human/Robot Collaboration
Efficient Plant October 2017 - 35
Efficient Plant October 2017 - Note These Causes Of Motor Failure
Efficient Plant October 2017 - 37
Efficient Plant October 2017 - 38
Efficient Plant October 2017 - Reliable Pumping Supplement: Food & Beverage Processing
Efficient Plant October 2017 - 40
Efficient Plant October 2017 - 41
Efficient Plant October 2017 - 42
Efficient Plant October 2017 - 43
Efficient Plant October 2017 - 44
Efficient Plant October 2017 - 45
Efficient Plant October 2017 - 46
Efficient Plant October 2017 - On The Floor
Efficient Plant October 2017 - 48
Efficient Plant October 2017 - Uptime
Efficient Plant October 2017 - IIoT
Efficient Plant October 2017 - Pump 'Cavitation' Might Not Be
Efficient Plant October 2017 - Never Too Late For Basic Navigation
Efficient Plant October 2017 - Audit or Assess? Understand The Difference
Efficient Plant October 2017 - A Perfect Pair: Barcodes and Your CMMS
Efficient Plant October 2017 - Lubrication Strategies
Efficient Plant October 2017 - 56
Efficient Plant October 2017 - 57
Efficient Plant October 2017 - 58
Efficient Plant October 2017 - 59
Efficient Plant October 2017 - Products
Efficient Plant October 2017 - 61
Efficient Plant October 2017 - Ad Index
Efficient Plant October 2017 - Showcase
Efficient Plant October 2017 - Efficiency Insight
Efficient Plant October 2017 - Cover3
Efficient Plant October 2017 - Cover4
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